Continuous casting machine

ABSTRACT

Continuous casting machine for the continuous casting of molten steel into a cast product, comprising a mold in which the molten steel is poured through an exit port of a nozzle, forming a bath of molten metal, and in which at least part of the metal is solidified, whereby the continuous casting machine is provided with control means for controlling the flow of molten steel and operative on the molten steel after entering the mold such that the flow pattern of the molten steel in the mold is basically symmetrical with respect to at least one plane of symmetry of the mold.

TECHNICAL FIELD

The invention relates to a continuous casting machine for the continuouscasting of molten metal in particular molten steel into a cast product,comprising a mould in which the molten metal is poured through an exitport of pouring means, forming a bath of molten metal, and in which atleast part of the metal is solidified, to a mould suitable for suchcontinuous casting machine and to a method for the operation thereof.

BACKGROUND ART

A continuous casting machine as referred to in this specification may beany of the known continuous casting machines such as a conventionalcasting machine for casting slabs having a thickness of about 250 mm ora thin slab casting machine for casting slabs having a thickness ofabout 150 mm or less e.g. in the range 50-100 mm.

Although not restricted to thin slab casting machines, in particular insuch machines where the velocity at which the metal enters into themould is high, the problem of unstable and/or unsymmetrical flow of themolten metal in the mould occurs. Most commonly, molten metal is pouredfrom a tundish into the mould through a submerged entry nozzle aspouring means connected to the tundish and reaching into the mould. Thecenter line of the nozzle generally corresponds with the center line ofthe mould.

A continuous casting machine of the referred type is well known in theart e.g. from WO 95/20445. A mould and a nozzle suitable for such acontinuous casting machine are known from WO 95/20443. A furtherembodiment of a nozzle is known from EP 0 685 282.

In practice it has shown that the molten metal after entering the mouldforms recirculations of unequal magnitude and shape. In the case of asingle exit port of the nozzle two recirculations develop in thevertical plane on either side of the nozzle: a smaller one and a largeone. The recirculations extend to the meniscus and cause a disturbancethereof, which disturbance is different for each of the tworecircula-tions. The heat transfer by the circulating molten metal tothe casting powder, floating on the surface of the molten bath, andtherefore the temperature of the casting powder is different for the tworecirculations. Consequently the effect of the casting powder on theheat transfer of the molten metal to the chilled walls of the mould isnot uniform. The same applies to the lubricating effect of the castingpowder between the walls of the mould and the metal. The recirculationsmay also lead to entrapment of casting powder and other inclusions intothe bath of molten metal. The resulting effect, apart from surface andbulk defects, is that the cast thin slab is not uniform in temperatureand because of the unpredictability of the position of each of therecirculations, the temperature distribution is not predictableultimately resulting in a non-uniform thickness, or in other wordsshape-defects, of the cast slab.

In modern steel making plants wherein in a continuous or semi-continuousprocess steel is cast, hot-rolled and in some cases ferriticly rolled,there is no or only a very limited possibility of correction of theshape of the cast slab. Therefore shape control in this type of plant isa particular problem.

Although the problem of unstable and unsymmetrical flow in the mould hasbeen elucidated with regard to thin slab casting, the problem alsooccurs in thick slab casting machines.

A direction in which in the prior art a solution was sought was theshape of the nozzle and of the exit ports thereof. Numerous proposalsfor the shape of the exit port, its angle relation to the longitudinalaxis of the nozzle and the shape of the bottom of the nozzle were made.In thin slabs this necessitated a funnel shape of the mould.

Following this direction has not led to a satisfactory solution of theabove-mentioned problems, in particular not to a solution suitable forthe various casting conditions connected with various steel grades andsizes of the cast product.

SUMMARY OF THE INVENTION

An object of the invention is to provide a continuous casting machinewith which these problems can be obviated or at least largely reducedand with which also other advantages can be obtained.

This object is reached with a continuous casting machine that ischaracterized in that, it is provided with control means for controllingand/or steering the flow of molten metal and operative on the moltenmetal after entering the mould such that the flow pattern of the moltenmetal in the mould is basically symmetrical with respect to at least oneplane of symmetry of the mould.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated by a description of a prior artapparatus and embodiments of the invention which are not limiting andare described with references to the accompanying drawings, in which

FIG. 1 shows diagrammatically the flow pattern in a model of the priorart apparatus, and

FIGS. 2-7 show diagrammatically the flow patterns obtained in a model ofvarious embodiments of the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention starts from the idea that the desired symmetry andstability are very difficult to achieve because the flow of molten metaland its behavior in the mould depends on many factors such astemperature and chemical composition of the molten metal, irregularitiesin the shape of the nozzle and changes therein during its lifetimebecause of wear and clogging, temperature gradients over the cooledwalls of the mould, deviation in the shape of the mould. All thesefactors influence the flow in the mould and because each of thesefactors is difficult to predict or control, the flow is difficult topredict or control by selecting the shape of the nozzle.

According to the invention, control means are provided that cause asymmetrical flow or in other words, cause symmetrical and basicallyidentical recirculations in the mould and eventually in the notsolidified portion of the cast slab, by controlling and or steering theflow of the molten metal after it has entered the mould through thenozzle.

According to the invention, unsymmetrical or unstable behavior of theflow of molten metal is not primarily sought to be corrected byselecting the shape of the nozzle and its exit port or ports but byinfluencing the resulting flow of the metal in the mould and eventuallyin the non-solidified portion of the cast slab.

A simple contactless and reliable embodiment of the invention ischaracterized in that the control means comprise at least one magneticbrake apparatus preferably one electro magnetic brake apparatus.

Electro magnetic brakes for performing a stirring or braking action on amolten metal flow are well known in the art and have proven to be areliable piece of equipment. In the known application as disclosed ine.g. EP 0 040 383 and EP 0 092 126 the electromagnetic brakes is usedfor stirring a bath of molten metal.

Electromagnetic stirrers are used for stirring the liquid metal betweensolidified dendritic solid crystals to remelt these crystals locallyalong the long axes and to form equiaxed shaped solidified crystals. Thevelocity of the liquid metal leaving the exit port of the entry nozzleis 10 to 100 times the casting speed. Electromagnetic brakes are used tobrake this high velocity flow of liquid metal entering the mould toprevent deep penetration of the inflowing liquid metal, therebypreventing deep penetration of unwanted inclusions. Despite thebeneficial effects of electromagnetic stirrers or brakes, the flow ofliquid metal in the mould is not acceptable in view of instability andasymmetry. These unwanted phenomena are not prevented with theelectromagnetic brakes and stirrers due to the practical operation.

Although static magnetic brakes are suitable it is preferred to useelectromagnetic brakes because of the obtainable higher magneticinduction and the simplicity of controlling the magnetic induction bychanging the current in the induction coils, in particular DC- or lowfrequency operated electromagnetic brakes.

According to the invention the control means, in this embodiment throughthe generation of an electromagnetic force field, effectively obstruct aperiodic oscillation phenomena of liquid metal and an asymmetric flow inthe mould, resulting in a very stable molten bath surface even in acondition of high casting speed of 2:0 m/min or more for conventionalcontinuous casting machines and 4:0 m/min or more for thin slab casters,leading to a very sound and uniform solidified shell of solidified metalin the mould. When for some reason an asymmetry in the flow develops,there is an inequality in velocity of the flowing metal. Since thebraking effect depends on the velocity the effect is to equalise theasymmetry by obstructing the higher velocity flow. Therefore the controlmeans cause the recirculation to be basically equal and stable. Theproductivity of the continuous casting machine, in other words theeconomics, is dependent on the casting speed and can be substantiallyincreased using the invention.

A very efficient embodiment of the invention is characterized in thatthe magnetic brake apparatus comprises two sets of magnetic brakingpoles spaced apart and operative in a braking way in a directionbasically perpendicular to the direction of the flow of molten metalentering the mould through the exit port.

In this embodiment an essential portion of the main flow can flow,unobstructed, through the space between the two sets of poles. The outerportions of the flow pass through the magnetic brakes and are braked.Because unsymmetry in flow entails inequality in velocity and becausethe braking effect depends on the velocity of the molten metal passingthe brake, the brake has an equalizing effect that prevents unsymmetryto occur and remedies occurring unsymmetries. Because of the simplicityof the construction this embodiment is easy to install and operate.Preferably each set of poles has a main distribution of the magneticfield perpendicular to the flow of molten metal entering the mould.

A simple and for general purpose application adequate embodiment of theinvention is characterized in that the control means are positionedsymmetrically with respect to the exit port of the pouring means.

The control means operate very efficiently in an embodiment of theinvention that is characterized in that the control means extend in adirection basically perpendicular to the direction of a flow of moltenmetal entering the mould through the exit port.

In order to allow certain amount of recirculation and flow along theside-walls of the mould a further embodiment is characterized in thatthe control means are operative within a range between ⅛ and ⅞ of thewidth of the mould. This embodiment allows for sufficient flow of moltenmetal to the meniscus while stabilizing the remaining flow.

Surprisingly good effects can be obtained with an embodiment of theinvention that is characterized by the control means comprisingseparating means for separating the flow of metal entering the mould inat least two subflows and for obstructing flow from one subflow to asecond subflow in both parallel and funnel shaped mould.

The control means in principle divides the main flow of molten metalinto two subflows in general of recirculation-shape, of equal magnitude.Unsymmetry means that one recirculation differs in magnitude from theother recirculation, unsymmetry therefore means that molten metal shouldpass the control means. Since such passage is obstructed by the controlmeans, the recirculations and therefore the flow in the mould arebasically equal and stable.

Preferably, the separating means comprise at least one set of magneticpoles, more preferably a set of electromagnetic poles. In a veryeffective embodiment the separating means is 1.5 to 10 times longer inthe direction of casting than in the direction perpendicular thereto,i.e. the width of the mould.

Preferably the control means extend mainly perpendicular with respect tothe flow of the molten metal. Preferably the control means are operativeonly over part of the longest side i.e. width of the mould, preferablybetween ⅛ and ⅞ thereof, each pole resulting in a main distribution ofthe magnetic field strength perpendicular to the flow of the moltenmetal entering the mould. Such control means as magnetic brake brakesand equalizes, due to the velocity dependency of the braking action, themain flow while giving a circulating flow the possibility to extend tothe meniscus for the desired heat transfer. High velocity and disturbingrecirculations occurring at the outer ends of the magnetic brakes passthrough the brakes and are efficiently braked and reduced.

In general, as a consequence of the symmetrical flow in the mould, thevelocity of occurring recirculations and the velocity at the meniscus ofthe mould both are relative low as compared with the situation known inthe prior art.

To reduce the velocity at the meniscus still further, another embodimentof the continuous casting machine according to the invention ischaracterized in that the continuous casting machine is provided withbraking means for lowering the velocity of the molten metal flowing atthe meniscus of the bath of molten metal in the mould.

In certain applications a still smaller velocity at the meniscus isrequired, mainly to prevent disturbance of the meniscus and entrapmentof particles of casting powder in the molten metal. With this embodimentthe velocity at the meniscus can be reduced without essentiallyinfluencing the equalizing and stabilizing effect of the control means.

A very efficient, reliable and easy-to-operate braking means ischaracterized in that, the braking means comprise at least two magneticbrakes preferably two electro magnetic brakes positioned symmetricallywith respect to at least one plane of symmetry of the mould andoperative on the flow of metal directed to the meniscus of the moltenmetal. The recirculations occurring in the mould are directed upwardlynear the short walls of the mould. Placing the braking means at thisposition, were the velocity is relatively high, a particular efficientbraking effect is obtained with magnetic brakes.

Preferably the position of the control means is variable with respect tothe mould. With this embodiment it is possible to place the controlmeans in an optimum position in dependency of the mould and nozzle used.It is even possible to adapt the position to varying process conditions,while casting.

Preferably the position of the braking means is variable with respect tothe mould. Also with this embodiment, an optimum position of the brakingmeans in dependency of mould, nozzle and process conditions can bechosen and maintained even when process conditions vary.

The invention is also embodied in a mould provided with control meansaccording to the invention and the further embodiments thereof and in amould suitable for operation with such control means.

The invention is further embodied in a method for casting steel using acontinuous casting machine according to the invention and embodimentsthereof.

In a preferred embodiment the method is characterized in that theoperation and/or position of the control means and/or brake means isselected in dependence of the temperature of the molten metal in themeniscus area.

A still further embodiment is characterized in that the operation and/orposition of the control means and/or brake means is selected independence of the flow characteristics of the nozzle in the mould.

DESCRIPTION OF EXAMPLES AND DRAWINGS

The object and other advantages of the present invention will beillustrated by the following description of various embodiments and testresults which are not-limitative and are described with reference to theaccompanying drawings. In the tables V_(mean) means the mean measuredvelocity at the meniscus.

In each of the figures identical numerals refer to identical items oritems with corresponding functions. In each figure the dotted lines andthe arrows therein indicate the direction of the flow of the moltenmetal.

The figures show the result of experiments conducted in a water modelsimulating the mould wherein water is used to simulate molten steel. Itis known in the art that such modelling gives a very good representationof the actual behavior of molten steel is a mould. The water model has arectangular cross-section of sizes 1500 mm width and 100 mm thickness inFIG. 1-6.

FIG. 1 shows the flow pattern as occurs in the prior art apparatus. Theflow is highly unsymmetrical. The measured velocities are shown in thefollowing table.

A V_(mean) [cm/s] mm left right 30 7

FIG. 2 shows the flow pattern wherein control means are applied to themould, the control means being for example a magnetic brake simulated bya mesh-type restriction. The letter A designates the distance betweenthe exit port of the entry nozzle and the control means. Part of thewater passes, braked, the control means, part is deflected upwardly andcauses the desired heat flow to the surface of the bath. At the end ofthe control means, small recirculations occur which are effectivelybraked by the control means.

The results are summarized in the following table which shows that asubstantial improvement in symmetry is obtained.

A V_(mean) [cm/s] mm left right 100 15 13 200 16 15 300 19 16 400 22 18

FIG. 3 shows the flow pattern obtained with another embodiment of theinvention. The magnetic brakes comprises two sets of poles spaced apartin a direction basically perpendicular to the direction of the flow ofmolten metal. The center position of the flow passes the brakeunobstructed. The side portion, which cause the recirculations arebraked and equalized leading to a symmetrical and relative low velocityof the recirculations. The measured results are shown in the followingtable.

A V_(mean) [cm/s] mm left right 200 10 9

FIG. 4 shows a further embodiment wherein the control means compriseseparating means embodied in a vertically placed magnetic brakes assimulated by a mesh-type control means, acting as an obstruction.

Surprisingly this embodiment has proven to be very effective. Theoperation is considered to be as follows: the control means splits themain flow in two subflows. Each subflow forming a recirculation. Oncethe main flow has been split in two symmetrically operatingrecirculations, instability and unsymmetry is prevented by theobstruction effect of the control means. The- splitting effect initiatesthe recirculations which prevent that the main flow enters deep into thebath and might thereby entail unwanted inclusions deep into the bathwhere they might be entrapped and included in the solidified metal suchas steel. Entrapped inclusions may lead to serious defects in the finalproduct.

It has been found that the operation of this embodiment is relativeinsensitive to the position of the control means relative to the entrynozzle in any direction. Also therefore this embodiment is veryeffective.

The obtained results are shown in the following table.

A V_(mean) [cm/s] mm left right 150 42 38 300 42 37

A further improvement can be obtained with an embodiment as shown inFIG. 5 which shows braking means for lowering the velocity of theflowing water at the meniscus of the bath. As can be seen from FIG. 4,the velocity at the surface is relative high. Such high velocity maycause a disturbance at the meniscus resulting in entrapment of meltingpowder particles such as in case of a steel bath. With the embodiment ofFIG. 5 the velocity at the surface of the bath can be reduced to safevalues without the risk of freezing of the meniscus. The measurementresults are shown in the following table.

A V_(mean) [cm/s] mm left right 300 18 19

The surprising effect of the embodiment of FIG. 4 can be demonstrated bythe results obtained with the embodiment of FIG. 6. In FIG. 6 only onebrake of the embodiment of FIG. 5 is in operation, which leads to verydifferent conditions between the left side and the right side of themould. Despite this great disturbance, the two recirculations rotatesymmetrically with respect to the plane of symmetry through the centerline of the nozzle and the mould. The measured velocities at the surfaceof the bath are as follows:

A V_(mean) [cm/s] mm left right 300 16 36

FIG. 7 shows another embodiment of the invention, in this case appliedto a bifurcated nozzle and a funnel shaped mould. The casting speed wasraised to 8 m/min. For each of the two mainflows exiting the nozzle amagnetic brake, simulated by a mesh-type control means is provided. Byselecting the angle of the control means with respect to the directionof the main flow the relative magnitude of the upwardly directed flowand the downwardly directed flow components can be chosen. Further,control of the flow is possible by selecting the braking effect of themagnetic brake. This performance of this embodiment was measured bymeasuring the wave-height of the meniscus. Wave heights are equal forthe left side and the right side and can be as low as 3 mm.

What is claimed is:
 1. Continuous slab-casting machine for thecontinuous casting of molten metal into a cast slab-product, comprising:a mould with long sides and short sides for receiving the molten metalpoured through an exit port of a pouring means, forming a bath of moltenmetal, in which at least part of the metal is solidified and extractedfrom the mould in a casting direction; at least one magnetic brakeapparatus comprising, on each of the long sides of the mould arespective magnetic braking pole for operating in a braking way in adirection basically perpendicular to the direction of the flow of moltenmetal entering the mould through the exit port mainly in the castingdirection, and the at least one magnetic brake apparatus extends in thedirection of the flow of molten metal entering the mould, thus acting asa separating means for separating the flow of metal entering the mouldin two subflows and for obstructing flow from one subflow to a secondsubflow, wherein the at least one magnetic brake apparatus is positionedfor operating in a braking way on flow component of the molten insidethe would which deviate from a flow pattern of the molten metal in thewould which is basically symmetrical with respect to a plane of symmetryof the would transversely to its long sides, without substantiallybraking flow components of a symmetrical flow pattern, and each saidmagnetic brake apparatus consists of an electro magnetic brakeapparatus, wherein the separating means comprises at least one set ofelectromagnetic poles, wherein the dimension of the separating means inthe casting direction is a multiplying factor of 1.5 to 10 longer thanthat of transverse of the casting direction.
 2. Continuous castingmachine according to claim 1, wherein, the magnetic brake apparatuscomprises two sets of magnetic braking poles spaced apart and which arepositioned symmetrically with respect to the exit port of the pouringmeans.
 3. Continuous casting machine according to claim 1, wherein thepoles of the magnetic brake apparatus have a rectangular shape with along side and a short side, and are positioned in such a way that, inoperation, the long side is directed basically perpendicular to thedirection of a flow of molten metal entering the mould through the exitport.
 4. Continuous casting machine according to claim 1, wherein themagnetic brake apparatus is a means for operating within a range between⅛ and ⅞ of the width of the mould.
 5. Continuous casting machineaccording to claim 1 wherein, characterized in that, the continuouscasting machine further is provided with braking means for lowering thevelocity of the molten metal flowing at the meniscus of the bath ofmolten metal in the mould.
 6. Continuous casting machine according toclaim 1, wherein the braking means comprise at least two magnetic brakespositioned symmetrically with respect to at least plane of symmetry ofthe mould and operative on the flow of metal directed to the meniscus ofthe molten metal.
 7. Continuous casting machine according to claim 5,wherein the braking means is movable such that the position of thebraking means is variable with respect to the mould.
 8. Continuouscasting machine according to claim 1, wherein the magnetic brakeapparatus is movable such that the position of the magnetic brakeapparatus is variable with respect to the mould.
 9. Continuous castingmachine according to claim 6, wherein the braking means is movable suchthat the position of the braking means is variable with respect to themould.
 10. The continuous slab-casting machine of claim 1, wherein themachine is for the continuous casting of molten steel.
 11. Method forcasting metal using a continuous casting machine according to claim 1.12. Method according to claim 11, wherein the magnetic brake apparatusis movable and the operation and/or position of the magnetic brakeapparatus is selected in dependence of the temperature of the moltenmetal in the meniscus area.
 13. The method of claim 11, wherein themetal is steel.